We are thinking of going to Rapperswil from Lucerne as a "no mountain due to weather" day alternative next June. The faster route is on the Sudostbahn vs the Swiss Rail System but we will have Swiss Travel Passes. Does the pass cover the Sudostbahn? I can't find any information on this by googling. Help appreciated.

Presume you mean travelling on the Voralpen Express (operated by Südostbahn) to Rapperswil. Yes that is covered by the Swiss Travel Pass. We took the Voralpen from Lucerne to St Gallen last June. In fact we took the Voralpen to the Swiss Transport Museum and to Arth-Goldhau to connect to Rigi. The platform (#1) was about 50 meters from our hotel (Continental Park) entrance - very convenient.

Great! You are a fountain of information. I try to do all my own research first without asking for help but sometimes the answers are just elusive. But I can always count on MySwissAlps when that happens. I wish every country I visited had such a fantastic website!